A Tale of Two Moons: Global Crater Databases for Saturn’s Moons Dione and Tethys

USGS SIM geologic maps are being produced for Saturn’s moons Dione and Tethys (Martin et al., 2023; White et al., 2024) at 1:5M scale. As part of these mapping efforts, two global crater databases for crater diameters (D) larger than 3 km on Dione and 4 km on Tethys are being compiled, although completeness at the smallest diameters are dependent on imaging in the mosaics. We are using the latest grayscale mosaics produced with an equatorial resolution of 153 m/pixel by the Cassini Team for Dione (e.g., Roatsch et al., 2013) and an equatorial resolution of 250 m/pix by Paul Schenk for Tethys. While all latitudes and longitudes are imaged for these mosaics, the imaging quality is not consistent. In some areas, the imaging has a slightly lower resolution, has solar incidence angles outside of the ideal for recognizing craters, 74-82° (Robbins et al., 2025), and/or has distortion associated with high emission angles. When the databases are complete, maps of where the imaging is outside of ideal for each mosaic will be provided (e.g., Fig. 1). Both databases start with crater measurements from previous work: Dione (Kirchoff & Schenk, 2015) and Tethys (Kirchoff & Schenk, 2010). The previous databases were compiled before Cassini completed its final flyby of each moon in the later part of Cassini’s Soltice Mission (2014-2016), which provided new, improved imaging for portions of each moon that are now being used in the new mosaics. We also lower the completion diameter for Dione from 4 km to 3 km. Furthermore, beyond the crater average diameter and central latitude/longitude included in the previous databases, the new databases include elliptical measurements (short & long axes, long axis orientation), the confidence that a feature is a crater, degradation state, whether the crater is in a cluster/chain, if the crater is polygonal (i.e., has a straight edge), if the crater interacts with tectonics, and morphology (e.g., central peaks, transitional, ejecta presence and type). The databases will be used to assign absolute model ages to mapped units, and also to help define the contacts of regional units. Here we present the latest progress for each database (Figs. 2, 3) and discuss comparisons.

References: Martin, E. S., et al. (2023). Progress on 1:5M Global Geologic Map of Saturn’s Moon Dione. 54th Lunar Planet. Sci. Conf., Abst. #1691. White, O.L., et al. (2024). A Forthcoming Global Geologic Map of Tethys. Annual Meeting of Planetary Geologic Mappers, Abst. #7007. Roatsch, Th., et al. (2013). Recent Improvements of the Saturnian Satellites Atlases: Mimas, Enceladus, and Dione. Planet. Space Sci., 77, 118–25, doi.org/10.1016/j.pss.2012.02.016. Robbins, S. J., et al. (2025). Crater Detection Dependence on Resolution, Incidence Angle, Emission Angle, and Phase Angle. Geophys. Res. Lett., 52, e2024GL110570, doi.org/10.1029/2024GL110570. Kirchoff, M. R. & P. Schenk (2015). Dione’s Resurfacing History as Determined from a Global Impact Crater Database. Icarus, 256, 78–89, doi.org/10.1016/j.icarus.2015.04.010. Kirchoff, M. R., & P. Schenk (2010). Impact Cratering Records of the Mid-Sized, Icy Saturnian Satellites. Icarus, 206, 485–97, doi.org/10.1016/j.icarus.2009.12.007.

Figure 1. Example of outlines of non-ideal imaging in the mosaic for Dione. Light pink – heavy shadows, dark pink – heavy shadows and stretched, yellow – low resolution, green – low resolution and stretched, blue-green – stretched, white – low sun angle, olive green – low sun angle and low resolution, lavender – low sun angle and stretched.

Figure 2. Current progress on Dione global crater database. Purple circles denote completed craters for D ≥ 3 km. Black circles denote completed craters for D < 3 km. White circles are craters in progress, mostly from the previous database produced from Kirchoff & Schenk (2015).

Figure 3. Current progress on Tethys global crater database. Colors are as in Fig. 2, but for D = 4 km and previous data is from Kirchoff & Schenk (2010).